It is desirable for electronic circuits to maintain a constant performance output irrespective of temperature. Not only do external environmental temperatures of an electronic circuit fluctuate but electronic circuits generate thermal energy which increases the internal temperature of the circuit and affects its performance. As an example, it is well known that the output of current sources and current mirrors vary with temperature. The output current of these current sources, moreover, may drive or bias loads located on a separate integrated circuit or chip which may have a response to temperature change that has been characterized. Such an off chip load is the vertical cavity surface emitting laser (VCSEL), a semiconductor laser which emits light parallel to the direction of the optical cavity. For these applications, a constant current source having a positive and/or a negative coefficient of temperature compensation is desirable. With such a constant current source, a new temperature coefficient can be selected in the current driver by changing the input if the temperature coefficients of all possible loads are not equal. If the temperature coefficient of the load is unknown at the time of manufacture and has to be characterized, moreover, flexibility to compensate for the temperature response is essential.
A key concept of a constant current source circuit is that the magnitude of the current does not change; rather the temperature coefficient associated with the current varies. As an example, if the load driven by the output current is a VCSEL and if the optical power output of the VCSEL has a negative optical power temperature coefficient, then the optical power output decreases with increasing temperature at constant current. To maintain constant optical output power throughout a temperature range, the VCSEL with the negative temperature coefficient has to be compensated by additional current from a current source having a positive temperature coefficient. If the temperature coefficient of the VCSEL and the current source temperature coefficient match in magnitude, but are opposite in sign, the optical output of the VCSEL will remain constant.
Several techniques may be used to compensate for temperature variations of a constant current source or mirror. A bandgap reference may be used to obtain a current having a zero temperature coefficient in that the current does not change as a function of temperature. A constant current source having a negative temperature coefficient, or a constant current source having a positive temperature coefficient may be used for compensation. In any of these three temperature compensation circuits, the magnitude of the current variation per degree change in temperature, also called the coefficient of temperature compensation or simply the temperature coefficient is permanently set by choosing semiconductor device dimensions, i.e., emitter widths, resistor values, or MOSFET device dimensions. Once the temperature compensation circuit is manufactured, the temperature coefficient cannot be changed. These techniques, therefore, are not suitably responsive to match the temperature coefficients of many different loads.
Several methods exist to generate currents with high temperature dependence. These generally involve using the temperature dependence of a voltage difference between the base and the emitter of a bipolar transistor or the temperature dependence of the threshold voltage of a field effect transistor. To generate a current having a larger temperature coefficient than one generated from a single bipolar or field effect transistor, two currents with opposite temperature coefficients can be subtracted from one another. Subtracting a first current with a negative temperature coefficient from a second current with a positive temperature coefficient results in a third current with a positive temperature coefficient that is larger than the temperature coefficient of the second current. Similarly, subtracting a first current with a positive temperature coefficient from a second current with a negative temperature coefficient results in a third current with a negative temperature coefficient that is larger than the temperature coefficient of the second current. Two currents can be generated with this method that have equal but opposite temperature coefficients; however, the process tolerance of these two temperature coefficients make this method inappropriate under certain circumstances such as for use in the digitally controlled reference of FIG. 1 as will be discussed because the result of two currents that are added or subtracted is very dependent on process tolerance.
It is thus an object of the present invention to provide a dual current source with equal and opposite temperature coefficients that are independent of power supply and process tolerances.